1. Field of the Invention
The present invention generally relates to steam turbines. More specifically, the present invention relates to an apparatus and method for reducing relative motion between the blades of a turbine.
2. Description of the Prior Art
In turbines, e.g., steam turbines, a plurality of rotating arrays of foils or blades are arranged circumferentially about a rotor. Reaction of steam or gases against the blades produces rotation of the rotor and associated blade arrays. The forces acting on these rotating blades, including centrifugal forces caused by rotation, tend to throw the blades radially outward and generate large forces on the blade attachment structure. In many turbines, the attachment structure comprises a fir tree-shaped root structure that slides into a mating slot or groove in the rotor. When the blade is properly positioned, lugs extending from opposing sides of the root structure share equally in retaining a blade to the rotor; however, if a blade is aligned off of a radial line of the rotor through the center of the rotor groove, lugs on one side of the root structure may support more force than those on the opposite side, leading to potential overstress conditions. Accordingly, it is desirable to assure that turbine blades are aligned with their respective centerlines oriented along axial radial lines of the turbine rotor.
Such arrays of rotating blades are often joined together at their tips by a shroud ring that is normally riveted to the blade via a tenon made integral with the blade. The tenon, being an abrupt change in cross section of the blade, is subject to higher stresses due to bending moments imposed by the shroud ring and also provides crevices wherein corrosion products are accumulated. However, the shroud ring also provides a steam seal over the top of the blade as well as reducing blade vibration to some extent. One prior art turbine blade that eliminates the problems of tenon stress and corrosion is disclosed in U.S. Pat. No. 4,533,298, issued to Albert J. Partington et al on Aug. 6, 1985, assigned to the assignee of the present invention, and incorporated herein by reference.
Partington et al teach a plurality of rotatable blades disposed in a circular array, each blade comprising a root portion, which fastens the blades to the rotor, an air foil-shaped blade portion having a leading edge and a trailing edge, and a shroud portion made integral with the blade portion and disposed on the radially outer end of the blade portion. The shroud portion has a leading planar surface and a trailing surface, the trailing planar surface being disposed generally parallel to an axial radial plane passing through the central portion of the root portion, and the leading planar surface, if extended, forming an angle with the radial axial plane passing through the center of the root portion generally equal in degrees to 360 divided by the number of blades forming the circular array. A method for assembling such turbine blades is disclosed in U.S. Pat. No. 4,602,412, also issued to Partington et al on July 29, 1986, assigned to the assignee of the present invention and incorporated herein by reference.
Other commonly assigned copending applications reflect the current state of the art with respect to shroud arrays. Reference is thus made to commonly assigned copending application Ser. No. 018,321, filed Feb. 24, 1987 and to commonly assigned copending application Ser. No. 53,300, filed May 22, 1987.
In typical steam turbines, the rotor material differs from the blade material, with integrally formed shrouds being comprised of the same material as the blades. The coefficient of thermal expansion of the rotor material is usually greater than that of the integrally shrouded blades. Thus, if the integrally shrouded blades are assembled cold with a minimum of clearance between the mating shrouds, then, upon heating to a higher operating temperature, such as is experienced in a high or intermediate pressure turbine, a gap will develop between adjacent shrouds due to the different coefficients of thermal expansion between the rotor and the integrally shrouded blades. This gap can lead to vibration of the blades, during full speed operation.
Accordingly, there is a need for an apparatus and method of controlling the shroud gap for integrally shrouded blades at higher operating temperature conditions to minimize the vibration of the blades during full speed operation.